Investment casting, which is also known as lost wax, lost pattern, and precision casting, is used to produce high quality jointless metal articles that meet relatively close dimensional tolerances. Typically, an investment casting is made by first fabricating a thin-walled ceramic mold as a negative of the article to be made. Molten metal is then introduced to this mold, which is also known as an investment casting shell. Shells are constructed by first making a facsimile or pattern from a disposable substrate of the metal object to be made by investment casting. Suitable meltable substrate materials include, for example, wax, polystyrene, or some plastics. Other disposable materials that vaporize or burn off completely are also used to form these patterns.
A ceramic shell is then formed around the disposable pattern. This shell formation is accomplished first by dipping the pattern into slurry made of a mixture of liquid refractory binders such as colloidal silica or ethyl silicate, and a refractory powder such as quartz, fused silica, zircon, alumina, and aluminosilicate. Then, relatively coarser dry refractory grains, known as stucco, are sifted on the freshly dipped pattern and air-dried. The process of dipping, stuccoing, and air-drying is repeated until a desired thickness is achieved. The shells are built up to a thickness in the range of ⅛ to about ½ inch. No stucco is commonly applied to the final slurry coat, called the seal coat. The green shell is then thoroughly air-dried.
The disposable pattern is then removed using methods that do not exert excessive pressure to the green shell, which commonly include steam autoclaving and flash firing. During autoclaving and flash firing, the pattern is melted away leaving only the shell and any residual substrate material. The shell is then heated to a temperature high enough to combust the residual organic substrate material and sinter the ceramic mold. The hot mold is then filled with molten metal. Various methods are used to introduce molten metal into shells including gravity, pressure, vacuum, and centrifugal force. When the metal in the casting mold has solidified and the shell has cooled sufficiently, the ceramic mold is broken away and the casting is separated.
Investment casting has been practiced for generations. But its continuous growth stems from the demand for producing more complicated parts with intricate design. This demand is driving the industry to develop new ways to make investment casting shells that generate fewer defects and can be produced with higher efficiency and cost effectiveness. Such efforts have resulted in the development of engineered shell systems and the addition of polymers, fibers, and other additives that have significantly improved the quality of the shells produced today. For example, the wide distribution refractory flour system has increased the slurry solids content thereby reducing drying time with improved shell strength. Fiber addition made it possible to build corners and edges more uniformly. Polymer addition has reduced pre-fire shell cracking from drying and handling. Most of these improvements are concentrated on the refractory flour. With exception to polymer addition, the same binders have been used in investment casting for the last several decades.
Polymer, glass, and ceramic fibers currently used in investment casting shells help build uniform edges with virtually no increase in shell strength and permeability. The bond between these fibers and the matrix is primarily physical and relatively weak. The engineered flour systems often use wide size distribution flour particles, which allow raising the slurry solids and produce relatively dense shells with comparatively lower permeability. Currently, a small amount of coarser microsilica is added to build thicker shell and improve permeability, which diminishes some shell strength. Shell strength and permeability inversely affect shell physical properties.
There thus exists and industrial need for investment casting shells having increased shell strength and improved permeability.